The proper development of ovarian follicles is essential for reproductive success and health. This requires the ordered assembly of granulosa and theca cells around a single oocyte. The thecal cell compartment of ovarian follicles provides structural integrity as well as androgen substrate for granulosa cell estrogen production. As such, theca cells play an indispensable role in follicular development, growth and steroidogenesis. Virtually no information is available about the factors which stimulate thecal cell recruitment and growth around the developing follicle. Platelet derived growth factor (PDGF) is a potent mitogen and chemotactic agent for cells of mesenchymal origin, such as theca cells. PDGF acts by binding one of two receptors, PDGF-Ralpha or PDGF-Rbeta, leading to the activation of several signaling cascades important in regulating cell growth and cell motility. The preliminary data in this application demonstrate that granulosa cells produce PDGF while theca cells express only PDGF-Rbeta and respond to PDGF by increased cell growth and decreased steroidogenesis via a Ras - MEK - ERK dependent pathway. Beyond this, very little is known about the role of PDGF in ovarian and follicular development. The present application proposes to utilize several approaches to determine 1) the developmental expression pattern of PDGF and PDGF-Rbeta in the ovary; 2) the effect of loss of PDGF-Rbeta in ovarian and follicular development; 3) examine the mechanisms by which signaling cascades activated by PDGF inhibit luteinizing hormone-stimulated steroidogenesis and promote cell motility. In Specific Aim 1 immunoflourescence and immunohistochemistry will be utilized to explore the developmental expression pattern of the PDGF system in the mouse ovary from the perinatal period through sexual maturity. In Specific Aim 2 ovaries from PDGF- Rbeta null mice will be transplanted to wild type mice in order to the explore the role and physiological significance of the PDGF system in ovarian and follicular development. Finally, in Specific Aim 3, thecal cell cultures will be utilized to determine whether activation of the MEK - ERK signaling pathway inhibits LH-stimulated steroid secretion by down regulation of the LH receptor and/ or through the inhibition of expression of steroidogenic enzymes such as P450 side chain cleavage and 17 alpha hydroxylase. Additionally, the role of PDGF-stimulated phosphatidylinositol-3-kinase activation in thecal cell chemotaxis will be explored. The proposed research should provide valuable developmental, physiologic and mechanistic information on the role of platelet derived growth factor and its receptor, PDGF-Rbeta, in ovarian follicular development and function.